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Elite on the BBC Micro and NES

Combat demo: DrawScrollText

[NES version, Bank 6]

Name: DrawScrollText [Show more] Type: Subroutine Category: Combat demo Summary: Display a Star Wars scroll text Deep dive: The NES combat demo
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * ShowScrollText calls DrawScrollText

Arguments: A The number of lines in the middle part of the scroll text, which is the total number of text lines minus 4: * 2 for scrollText1 (6 lines) * 6 for scrollText2 and creditsText1 (10 lines) * 5 for creditsText2 (9 lines) * 3 for creditsText3 (7 lines)
.DrawScrollText PHA ; Store the number of lines in the scroll text on the ; stack so we can retrieve it later JSR CalculateGridLines ; Reset the line coordinate tables and populate them ; with the characters for the scroll text at (Y X), ; setting INF(1 0) to the scroll text in the process LDA #$28 ; Set the visible colour to orange ($28) so the scroll STA visibleColour ; text appears in this colour LDA #0 ; Clear bits 6 and 7 of allowInSystemJump to allow STA allowInSystemJump ; in-system jumps, so the call to UpdateIconBar displays ; the fast-forward icon (though choosing this in the ; demo doesn't do an in-system jump, but skips the rest ; of the demo instead) LDA #2 ; Set the scroll text speed to 2 (normal speed) STA scrollTextSpeed JSR UpdateIconBar_b3 ; Update the icon bar to show the correct buttons for ; the scroll text LDA #40 ; Tell the NMI handler to send nametable entries from STA firstNameTile ; tile 40 * 8 = 320 onwards (i.e. from the start of tile ; row 10) ; We now draw the scroll text and move it up the screen, ; which we do in three stages ; ; * Stage 1 moves the first few lines of the scroll ; text up the screen until the first line reaches ; the middle of the screen (i.e. just before it will ; start to disappear into the distance); stage 1 is ; always 81 frames long at normal speed ; ; * Stage 2 then draws the rest of the scroll text ; on-screen while moving everything up the screen, ; reusing lines in the line coordinate tables as ; they disappear into the distance; stage 2 is ; longer with longer scroll texts ; ; * Stage 3 takes over when everything has been drawn, ; and just concentrates on moving the scroll text ; into the distance without drawing anything new; ; stage 3 is always 48 frames long at normal speed ; ; We start with stage 1 LDA #160 ; Set the size of the scroll text to 160 to pass to STA scrollProgress ; DrawScrollFrames ; ; This equates to 81 frames at normal speed, with each ; frame taking scrollTextSpeed off the value of ; scrollProgress (i.e. subtracting 2), and only ; stopping when the subtraction goes past zero JSR DrawScrollFrames ; Draw the frames for stage 1, so the scroll text gets ; drawn and moves up the screen ; We now move on to stage 2 ; ; Stage 2 takes longer for longer scroll texts, and its ; length is based on the value of A passed to the ; routine (which contains the total number of text lines ; minus 4) ; ; Specifically, stage 2 loop around A times, with each ; loop taking a scrollProgress of 23 (which is 12 frames ; at normal speed) ; ; Each loop draws an extra line of text in the scroll ; text, and scrolls up by one line of text PLA ; Set LASCT to the value that we stored on the stack, so STA LASCT ; LASCT contains the number of lines in the scroll text .dscr1 LDA #23 ; Set the size of the scroll text to 23 to pass to STA scrollProgress ; DrawScrollFrames JSR ScrollTextUpScreen ; Scroll the scroll text up the screen by one full line ; of text JSR GRIDSET ; Call GRIDSET to populate the line coordinate tables at ; X1TB, Y1TB and X2TB (the TB tables) with the lines for ; the scroll text in INF(1 0) at offset XC JSR DrawScrollFrames ; Draw the frames for stage 2, so the scroll text gets ; drawn and moves up the screen by one text line DEC LASCT ; Loop back until we have done LASCT loops around the BNE dscr1 ; above ; We now move on to stage 3 ; ; Stage 3 loops around four times, with each loop taking ; a scrollProgress of 23 (which is 12 frames at normal ; speed), so that's a grand total of 48 frames at normal ; speed LDA #4 ; Set LASCT = 4 so we do the following loop four times STA LASCT .dscr2 LDA #23 ; Set the size of the scroll text to 23 to pass to STA scrollProgress ; DrawScrollFrames JSR ScrollTextUpScreen ; Scroll the scroll text up the screen by one full line ; of text JSR DrawScrollFrames ; Draw the frames for stage 3, so the scroll text moves ; off-screen one text line at a time DEC LASCT ; Loop back until we have done LASCT loops around the BNE dscr2 ; above ; The scroll text is now done and is no longer on-screen LDA #0 ; Reset the scroll speed to zero (though this isn't read STA scrollTextSpeed ; again, so this has no effect) LDA #$2C ; Set the visible colour back to cyan ($2C) STA visibleColour RTS ; Return from the subroutine